Source : Webster's Revised Unabridged Dictionary (1913)

Horizontality \Hor`i*zon*tal"i*ty\, n. [Cf. F.
   horizontalit['e].]
   The state or quality of being horizontal. --Kirwan.

Source : WordNet®

horizontality
     n : the quality of being parallel to the horizon; "houses with a
         pronounced horizontality"